A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In conventional lithographic apparatuses, an article, such as a substrate or a reticle, is clamped on an article support by a clamping force. The article support may comprise a plurality of supporting protrusions which define a support zone for the article. The support zone is typically planar, for example, the X-Y plane. Such supporting protrusions may be referred to in the art as “burls”. Conventional supporting protrusions are designed so that they reduce or prevent distortion of the article in the supporting plane. Conventionally, this is achieved by making the supporting protrusions as stiff as possible against (XY) movements in the plane of the support zone at the top of the protrusions. Conventional supporting protrusions are further designed in order to reduce or prevent distortions of the article out of the (XY) support plane, that is in the Z direction. Conventionally, this is achieved by making the supporting protrusions as stiff as possible against forces downwards along the Z-axis. In order to clamp the article to the article support, a vacuum may be applied to keep the article fixed on top of the supporting protrusions. In case of thermal load, the stiffness of the supporting protrusions, in particular in the (XY) direction defining the plane of the support zone, may not be not enough. As a result significant (XY) distortions of the article may arise in the plane of the support zone. In the case of the article being a substrate this may lead to overlay problems, and as a side effect also distortions out of the (XY) support plane may arise, which in the case of a substrate may lead to focusing problems. In particular, in the case where the article is a substrate, illumination induced substrate heating may result in the one or more of the problems outlined above. In particular, in high dose lithographic apparatuses, a substrate may absorb a lot of energy as a result of the exposure process. Unfortunately, the heat resistance of the supporting protrusions is typically so large that the heat absorbed by the substrate does not flow to the underlying substrate table (article support) fast enough. As a result, the substrate may still heat up locally, and therefore expand with respect to the substrate table. Conventional supporting protrusions of the substrate table try to reduce or prevent any distortions of the substrate in the (XY) support plane, but because the supporting protrusions (and the substrate table itself) have only a limited stiffness, the substrate may nevertheless distort significantly. For example, overlay effects of about 150 nanometers may occur. A side effect of the substrate table trying to, but not succeeding in, reducing or preventing distortions in the (XY) support plane is that the substrate may also be distorted out of the (XY) support plane, i.e. in the Z-direction, so that focus problems may result as well. Similar problems may occur in immersion lithography as well, as a result of contraction of substrates caused by cooling.